In fast, motor-driven watercrafts, particularly those having a surface drive, the hull is in a planing state at higher speeds, with the stern just lying partially in the water. When turning curves and during the related tilted position of the watercraft, the point of contact with the water shifts to the side surface of the bottom of the hull causing the force application of the driving resistance at the hull to change and generate a moment. From a critical control angle and the resultant tight turning radius the thus generated moment is so high that starting at a certain speed the watercraft is deflected from its path and may capsize.
The surface drive of a watercraft consists of at least one drive unit, in which the direction of the propulsion force is changed by a control unit, which mainly consists of a thrust tube that guides a propeller shaft and a control and trim actuator system. The thrust tube can be pivoted about a control angle and is connected with the stern of the watercraft and the drive shaft coming directly from the motor or a transmission downstream of the motor. The thrust tube is furthermore moved by the trim actuator system in the vertical direction and, to generate a change in direction of the watercraft, by the control actuator system in the horizontal pivot plane. The maximum freedom of movement in both planes is characterized by a maximally adjustable control angle and a trimming range. The control angle and the trim angle are the measure of the respective pivoting motion. The activation of the control and trim actuator system is accomplished via an electronic control unit, which electronically receives the desired control angle signal from the skipper. The trim angle can optionally be speed-dependently and/or rotation-dependently adjusted to an automatic mode of operation. In order to prevent excessive control angles at high speeds, the speed of a watercraft and the control angle are acquired by the electronic control unit, and a reduced maximum adjustable control angle is accordingly automatically determined. The control range is again correspondingly increased when the speed is reduced.
At high speeds in connection with a tight turning radius and/or a maximum control angle the watercraft takes up a significantly tilted position, whereby the described unstable driving condition need still not to have been reached. It is on the other hand possible that in a surface drive, which comprises at least two drive units, from a critically tilted position the propeller of the drive unit on the outside curve is no longer running in the water. The load and the rotational speed increase because of the missing or significantly reduced displacement of the propeller. The automatic trim adjustment cannot correct this condition because both drive units are mechanically connected, and the connection only permits minor interconnections of the drive units.
From JP 02-279495, a system is known where the maximum adjustable control angle in an outboard drive is mechanically reduced with increasing speed.
U.S. Pat. No. 6,843,195 B2 describes a control system for an outboard motor, where the quotient “realized control angle/control angle entered by the rudder” declines with increasing speed, so that with increasing speed turning the steering wheel at the outboard drive will result in a minor change of the control angle as at lower speeds. Moreover, a method for limiting the maximum adjustable control angle depending on the rotational speed and/or speed is disclosed.
Both systems only relate to outboard motors whose propeller and control arrangement are significantly different from a surface drive.